An instrument landing system (ILS) is a ground-based transmission system that provides runway approach information to aircraft. Some conventional systems comprise a localizer subsystem and a glide slope subsystem. The localizer subsystem provides lateral guidance to aircraft for tracking the runway centerline. The glide slope subsystem provides vertical guidance to aircraft for descending at the proper glide path.
Conventional localizer subsystems include a localizer antenna array that radiates a signal comprising an RF carrier amplitude modulated with equal amplitudes of 90 Hz and 150 Hz tones. The localizer receiver resides with the aircraft. The localizer radiation patterns are normally arranged such that the 90 Hz and 150 Hz tones will have equal levels when the receiving aircraft is on the runway centerline and extention thereof.
Similarly, conventional glide slope subsystems include a glide slope antenna array that radiates a signal comprising an RF carrier amplitude modulated with equal amplitudes of 90 Hz and 150 Hz tones. The glide slope resides with the aircraft. The glide slope radiation patterns are normally arranged such that the 90 Hz and 150 Hz tones will have equal levels when the receiving aircraft is descending at the desired glide path angle, which is typically referenced at three degrees.
However, some conventional instrument landing systems, including conventional localizer and glide slope systems, may be implemented on aircraft that generate propeller or rotor modulation effects and/or observe multipath effects, which can introduce errors into the ILS. For example, errors may be introduced when the propeller or rotor modulation frequency or the multipath frequency is approximately 90 Hz or 150 Hz (i.e., the same as either one of the localizer and glide slope tones of the ILS) or a subharmonic of 90 Hz or 150 Hz (e.g., 15, 30, 37.5, 45, or 75 Hz). Such errors can cause the aircraft to oscillate or fly an offset course.
Accordingly, it may be desirable to provide an instrument landing system capable of removing undesired modulation effects such as, for example, propeller or rotor modulation effects.